This invention relates to a conductive board spacer for fixing two opposing printed boards at a predetermined space while enhancing the function of the printed boards.
Conventionally, this conductive board spacer is composed of synthetic resins or metals.
Japanese Published Examined Patent Application No. 58-35386 and Japanese Published Examined Utility Model Application Nos. 59-27651 and 60-8462 disclose board spacers of synthetic resin having a predetermined length. The synthetic-resin spacers have a supporting member with both ends of the supporting member contacting printed boards. Locking members extend from the ends of the supporting member for engaging fixing holes in the printed boards. The locking members, which are arrowhead-shaped, pass through the holes, resiliently expand outward, return to their original configuration and become secured in the fixing holes. The synthetic-resin spacers thus secure the printed boards, and easily engage and disengage from the boards.
On the other hand, metallic spacers have a long, hexagonal bolt. The bolt engages screwed holes in the printed boards, and nuts screw onto the bolt, thus fixing the spacer to the printed boards. The spacers keep the printed boards a certain distance apart and equalize the ground potential between the printed boards. Signal conductors laid between the printed boards are thus effectively used.
However, the synthetic-resin spacers do not equalize the ground potential between the printed boards. On the other hand, since the metallic spacers must be fixed to or disengaged from the printed boards using the nuts, the application of the spacers is troublesome and time-consuming.
Conductive spacers, which have been developed to provide advantages of the abovementioned spacers, are molded as one piece from synthetic resin with conductive fillers such as metallic particles, having a particle size of several tens of microns, or carbon black.
However, the conductive spacers develop problems. The conductive filler provides conductivity to the conductive spacers. When the conductive filler is mixed into synthetic resin, the electric resistivity of the synthetic resin is reduced, but its mechanical strength is also reduced. The amount of conductive filler must be carefully regulated.
The optimum resistivity for grounding the printed boards each other is 5.times.10.sup.-5 ohm.cm or less. As the amount of the conductive filler is increased to provide optimum conductivity, the synthetic resin becomes brittle. Especially, the excess amount of the conductive filler deteriorates the strength and durability of thin portions of the conductive spacer which portions engage the fixing holes in the printed boards.